Toggle clamps have long been used as factory tooling components, for example, as part of fixtures which releasably hold workpieces during machining, assembly and similar operations. The clamps are adjustable for work-pieces of different thickness by screwing a clamping foot relative to a linkage-operated clamping arm or force-applying member. The member is set in clamping position by an over-center toggle link, manually or power operated by a handle or equivalent, on one of the linkage elements employed. The adjustment for thickness being made by screwing the clamping foot, the clamping arm itself has no substantial range of angular adjustment during clamping.
Since such toggle clamps may be used on opposite edges of workpieces to be assembled, the clamping arms, or force-applying members which extend over the workpieces for clamping, must be withdrawn completely out of the way of the workpieces to permit their removal from the fixture and the placement in the fixture of new workpieces. Accordingly, the linkage utilized in such toggle clamps must provide not merely for releasing the clamping, but in addition a wide range of angular movement for clearance, to permit such removal and placement of the workpieces.
By the term "toggle action" or "toggling" as used in such clamps and in the present invention, is meant using a link member with parallel pivot axes at its ends, in angular movement relative to a centerline between one of its ends, whose position is fixed, and a third parallel pivot axis. In such angular movement, as the other end of the link member approaches this centerline connecting the other two pivots, the link member is subject to intense axial loading, the reactions of which put the members of the linkage mechanism under such strain as to permit such other end of the link to cross the centerline to a "locked" position against a stop. In conventional toggle clamps, the clamping foot is screwed to adjust for thickness, so that the linkage always "toggles" in the same position.
In contrast, in the present invention the effective length of one of the linkage members is automatically changed to respond to the thickness of the workpiece to be clamped. This is done in a manner somewhat like that used in self-adjusting locking pliers. These are hand-held tools, as shown in U.S. Pat. No. 2,531,285 to Manspeaker and No. 3,600,986 to Baldwin. Such hand-held tools are, however, substantially different in their design criteria and manner of construction from the present invention. Hand-held tools are brought to the work and withdrawn from it; unlike toggle clamps for factory fixtures, they do not need and they are not provided with a substantial range of movement merely for clearance; once loosened from the article which they have theretofore clamped, they are simply taken away from it.
The locking pliers shown in these patents may be locked in self-adjusted position over a significant range of thicknesses. Their operating principle is this: one jaw of the pliers is integral with a fixed handle to which the other jaw is pivotally attached. The movable handle is pivotally attached to this other, or "moving", jaw. The fixed handle serves as a variable length linkage member; this handle has walls defining a slide path for a locking wedge assembly which carries the pivot at one end of a toggling link, whose other end is pivotally attached to the movable handle at a point spaced from its pivot to the moving jaw. A line connecting this pivot to the pivot carried by the wedge assembly is the centerline across which toggling action must take place. Until angular movement of the link causes its handle pivot to approach this centerline, a cam, formed on the link at its other end which pivots relative to the wedge assembly, holds the assembly unlocked. In toggling action, as the centerline is approached the cam permits the wedge assembly in the fixed handle to lock to its slide path, thus fixing the position of the pivot carried by the wedge assembly.
When the handles are squeezed, if nothing is between the jaws, closing movement of the moving jaw will draw its pivot to the movable handle sufficiently forward that the wedge assembly may remain in its most forward position along its slide path. Then, as the jaws close, the link moves angularly so that the cam at its wedge end effects clamping of the wedges in the slide path.
However, if the moving jaw closes against an object of substantial thickness, further squeezing the handles so drives the link that its end pivoted to the wedges will slide them up the handle, permitting the link to move angularly until the cam causes the wedges to clamp in their then established position along the slide path. When so clamped, toggling forces can be reacted; hence the jaws clamp securely as the handle-to-link pivot crosses such centerline.
To release the clamping, the initial movement of drawing the handles apart causes this pivot to re-cross the centerline; after this point, the wedge assembly releases its affixment to the slide path and a tension spring draws it forward, and also draws the movable jaw to open position and the handles farther from each other.